Absorbent articles intended for single use normally have a liquid-permeable coversheet, which is intended to face the body of the user when in use. Such a coversheet is often made of a nonwoven material, i.e. a fibre cloth in which the component fibres are bonded together in a way other than weaving.
It is also known to arrange a liquid transfer layer between the coversheet and an absorbent body comprised in an article. This should have the ability to quickly receive large amounts of liquid and spread the liquid, as well as temporarily store it before it is absorbed by the underlying absorbent body. This is important, especially for the thin, compressed absorbent bodies of today, which often have a high content of highly-absorbent polymeric material, so-called “superabsorbents”. The absorbent bodies of today certainly have high absorption capacity, but on the other hand, they often have a receiving speed which is too low to instantaneously cope with receiving the large amounts of liquid such as e.g. can be released upon urination in the space of a few seconds.
A porous, relatively thick liquid transfer layer, e.g. in the form of fibrous wadding, a bonded or unbonded carded fibre layer, or another type of fibre material has high instantaneous liquid-receiving capacity and can therefore temporarily store liquid until the absorbent core manages to absorb it. The same relationship also exists for many porous foam materials.
So that the absorbent article is able to receive repeated liquid volumes, it is important that the liquid transfer layer substantially manages to empty of liquid between each wetting. The open coarse capillary structure of the transfer layer thereby cooperates in a suitable fashion with the finer capillary and/or more hydrophilic absorbent body.
EP patent 0,685,214 is an example of a document which describes absorbent articles comprising a liquid transfer layer of the type described above. Further examples of documents which describe various types of liquid transfer layer are WO 99/49825 and WO 97/02133.
Documents EP 0,685,214, WO 97/02133 and WO 99/49825 also describe how liquid transfer between the outermost lying coversheet and the liquid transfer layer can be further improved. The documents describe how different coversheets and transfer layers can locally be bonded together, whereby the pore size of the liquid transfer layer is locally reduced in association with the joined areas. The reduction in pore size is a result of that, upon joining, one has also pressed the liquid transfer layer together around the joined areas. The pores in the liquid transfer layer have thereby been reduced so much that they have become smaller than the pores in the coversheet, whereby liquid moves from the coversheet to the liquid transfer layer through capillary action. Tests have also shown that effective transport of liquid stored in capillary form takes place in the coversheet in the direction of the joined areas, at the same time as liquid is emitted from the coversheet and is transported further into the liquid transfer layer around the joined areas.
This capillary effect means that only small amounts of liquid or no liquid at all, is bound in the capillaries of the coversheet, i.e. that layer which lies in contact with the skin of a user when in use. The result is an absorbent article which has an extremely dry contact surface against the skin of the wearer, after an article has received released bodily fluid from the wearer. The initially open porous structure of the liquid transfer layer is substantially maintained between the intermittently-arranged bonding areas.
It is also known to carry out intermittent joining of continuous material webs with ultrasound. For example, U.S. Pat. No. 3,939,033 describes the use of an ultrasound horn in combination with a rotating counter roller for joining continuous material webs in a particular pattern, in which the purpose of the pattern is to provide intermittent joining of the continuous material webs. The rotating counter roller comprises the pattern which is to be transferred to the laminate material which is to be joined by means of the ultrasound apparatus.
Ultrasound is an example of an especially suitable technique, as it only warms the material to be joined very locally, and leaves the remaining parts of the material unaffected by heat. Heat effects on those types of materials which are used for coversheets and liquid transfer layers affect the material in a negative way.
A well-functioning liquid transfer layer must have a certain volume so as to be able to temporarily store an instantaneous volume of liquid, which is why it is necessary that such layers have a certain thickness. On the other hand, it is not necessary that the liquid transfer layer extends across the entire surface of the absorbent article; rather it is sufficient that a necessary reception volume is located adjacent to the area of the wetting area of the absorbent article, i.e. that area which receives liquid upon use. To minimise material costs for an absorbent article, it is therefore well-known to design articles in which the liquid transfer layer only has extension in the central portions of the article in which liquid is usually received.
U.S. Pat. No. 4,908,026 describes examples of different designs, in which none of the layer covers the entire surface of the article which is intended to face the user when in use.
It is also desirable to create a liquid transfer layer which locally comprises such small pores that it preferentially absorbs liquid from a coversheet and which is furthermore arranged only in the central portions of the absorbent article, i.e. a combination of the above-described properties.
In summary, there is a desire to create a laminate comprising a substantially centrally-arranged liquid transfer layer which is joined to the coversheet in discrete bonding areas.
It is important that the discrete bonding sites between the coversheet and the liquid transfer layer are visible on the complete manufactured article, so that the absorbent article will give the user an impression of reliability. This requires that the bonding pattern should be visible from the side of the laminate which constitutes the coversheet, i.e. from that side of the laminate which is intended to face outwards when positioned on an absorbent article.
To create a visible pattern with the most commonly-occurring ultrasound technique, i.e. the technique described in U.S. Pat. No. 3,939,033, the rotating counter roller which comprises the pattern to be transferred to the laminate material must be located on the side of the laminate material from which the pattern is to be visible. The pattern is formed by pattern elements which project from the surface of the circular counter roller, whereby the pattern elements create corresponding permanent impressions in the laminate material. The rotating counter roller must therefore be positioned on the side of the laminate material comprising the coversheet, whereby the ultrasound horn must be positioned on the side of the laminate material where the liquid transfer layer is located. For laminate materials where both the coversheet and a liquid transfer layer comprise continuous material webs, this positioning of the ultrasound horn does not pose any problems.
For laminate materials in which the liquid transfer layer is formed from discrete material pieces, however, problems occur. The front edge of the discrete material pieces, i.e. that edge which is arranged forwards in the transport direction through the ultrasound apparatus, quite simply does not enter into the narrow slit between the coversheet of the laminate and the non-rotating ultrasound horn, but rather gets stuck in the slit and causes a blockage in the machinery.
In patent document WO 00/02727, one has solved this problem through stitching the front edge of each discrete material piece to the coversheet in a first reverse ultrasound operation, so as to thereby create the visible pattern in a second ultrasound operation. By the front edge of each discrete material piece is meant that edge which is oriented in the front edge of the material piece in the direction of movement.
By a first reverse ultrasound operation is meant an ultrasound operation in which the rotating counter roller is arranged on the same side of the laminate as the discrete material pieces, whereby the discrete material pieces automatically follow the rotating counter roller into the slit between the coversheet and the counter roller. In this first ultrasound operation, a substantially transverse joint is preferably provided between the coversheet and each material piece, whereby the joint is located in the front edge of each material piece in the direction of travel. In the second ultrasound operation, the discrete material pieces are located on the same side as the ultrasound horn, but as each material piece and coversheet are joined at the front edge of the material piece, no problem arises when the material piece is pressed into the narrow slit. Each discrete material piece is drawn into the slit by the continuous coversheet material. In that the counter roller comprising the desired pattern is arranged on the side of the laminate upon which the coversheet is located, the pattern will be visible from that side of the laminate which faces outwards on the complete manufactured absorbent article.
The ultrasound apparatus described in WO 00/02727 functions well, but is however unnecessarily expensive and complicated in many situations. This expensive and complicated apparatus can however be justified if the machine will primarily produce articles comprising discrete liquid transfer layers under long periods of time, without frequent changeover to different types of article.
In other machines, one wants to be more flexible and to be able to change the machine over more frequently for production of different types of article and, for instance, only manufacture absorbent articles comprising discrete liquid transfer layers or other types of discretely-arranged material layers for a short period of time.
The above-described two-step apparatus comprising one ultrasound unit for stitching the material piece and one ultrasound unit for pattern formation is both unnecessarily complicated and unnecessarily expensive for such machines. Due to the complexity of the two-step apparatus, the trimming period upon changeover to production of articles comprising liquid transfer layers is unnecessarily long and costly. Lost production volume due to long changeover times between types of article is also a negative factor for machines which are intended to alternate between different types of article. It is for instance common that one primarily produces articles without any liquid transfer layer and only provides articles with a liquid transfer layer under shorter periods.
There therefore remains a need for a laminate intended as a surface material for an absorbent article which has good liquid transfer properties and low rewet, which can be produced by an uncomplicated manufacturing process.